1. Field of the Invention
The present invention generally relates to the field of power supplies, and more particularly, to reducing audible noise in switch-mode power supplies that have variable switching frequency.
2. Description of the Prior Art
Today, the power supply industry is at the beginning of a major focus shift that puts efficiency improvements across the entire load range in the forefront of customers' performance requirements. This focus on efficiency has been prompted by economic reasons and environmental concerns caused by the continuous growth of the Internet infrastructure and a relatively low energy efficiency of its power delivery system. In fact, the environmental concerns have already prompted introduction of programs and initiatives aimed at reducing the energy waste in power supplies for data-processing applications by challenging power-supply manufacturers to improve efficiency of their products.
A switch-mode power supply comprises a switching power converter for regulating the supply's output voltage. The power converter comprises at least one switch, transformer and a controller that controls the switch according to a switching frequency. This type of power supply suffers from two types of power losses: switching loss and conduction loss. Switching loss is associated with the switching frequency such that this loss increases as switching frequency increases. Conduction loss is associated with current flows in the switch such that the conduction loss increases as the current through the switch increases.
Generally, efficiency across the entire load range requires balancing the switching and conduction losses. This is because efficiency at full load is predominantly determined by conduction losses of semiconductor and magnetic components, whereas efficiency at light/no load is for the most part determined by switching losses of semiconductors and core losses of magnetic components.
One type of switch-mode power supplies operates using a constant switching frequency and another type operates using variable switching frequencies. Regardless of the type, meeting the efficiency requirements at light/no load involves manipulating the switching frequency. In switching power supplies that employ constant switching frequency, cycle skipping (also called burst-mode operation) is employed for meeting the efficiency requirements. In variable switching frequency power supplies, the switching frequency continuously decreases as the load decreases. In some implementations, the burst mode of operation is also used in switch-mode power supplies with variable switching frequency.
However, when the switching frequency is adjusted to be within the upper audible switching frequency range of 200 Hz to 20 kHz, the transformer of the power converter could be excited to generate an unacceptable noise level. As further described below, the switching frequency has an upper audible frequency range and a lower audible frequency range of 20 Hz-200 Hz that is at the lower threshold of the audible range. If the audible noise is above a specified acceptable level, e.g., 25 dB(A)/20.0 uPa for external ac-dc power supplies, appropriate measures should be taken to reduce the audible noise. B. Weisner et al., in “Relevance of magnetostriction and forces for the generation of audible noise of transformer cores,” IEEE Trans. Magnetics, vol. 36, no. 5, September 2000, describe two separate parts for transformer noise that depend on different excitation mechanisms. The first and most dominant part of the transformer noise is caused by magnetization of the core, which is generally assumed to arise from magnetostriction. Under this mechanism, the core dimensions change when subjected to an applied magnetic field. The second part of the transformer noise is caused by electromagnetic forces created by the magnetic field of the currents in the transformer windings. Generally, magnetostriction can cause a mechanical interaction between the core and the windings that leads to a vibration. The mechanical vibration of the transformer is closely related to the magnetic flux swing.
Known methods for reducing the audible noise during burst-mode of operation are disclosed in 1) U.S. Pat. No. 6,477,066 to F. L'Hermite, entitled “Method and apparatus for reducing audible acoustical noise in a power supply transformer by shaping the waveform of a primary side inductor current,” 2) U.S. Pat. No. 6,525,514 to B. Balakrishnan, et all, entitled “Method and apparatus for reducing audio noise in a switching regulator,” and 3) U.S. Patent Application Pub. No. 2006/0171179 by J. W. Hall and C. Basso, entitled “Low audible noise power supply method and controller therefore.” U.S. Pat. No. 6,477,066 and U.S. Patent Application Pub. No. 2006/0171179 disclose reducing the audible noise in burst-mode of operation by shaping the envelope of the switch current pulses. U.S. Pat. No. 6,525,514 discloses reducing the audio noise by presetting the switching frequency values above the audible range and by decreasing the switch current limit as the load decreases until the lowest current limit has been reached. This limit is low enough that the flux density in the core of the transformer does not produce unacceptable audible noise. Specifically, in U.S. Pat. No. 6,525,514, constant switching frequency power converters with on/off feedback control and with a relatively complex cycle skipping algorithm are considered.
While the above methods apply to burst mode operation, in switch-mode power supplies with variable switching frequency, the audible noise can be completely eliminated by preventing the switching frequency from dropping below the upper threshold of the audible range. This can be achieved directly by limiting the maximum switching period or by limiting the maximum off time. The same can be achieved indirectly by monitoring the switching frequency to detect when the switching frequency drops close to the upper threshold of the audible range and, then, instantly increasing the switching frequency, as described in U.S. Pat. No. 7,202,609 to W. H. M Langeslag and J. W. Strijker, entitled “Noise reduction in a power converter.” It should be noted that U.S. Pat. No. 7,202,609 is directed towards the problem of reducing the audible noise in power converters with variable switching frequency. The disclosed arrangement in this patent cover a power supply design that decreases the switching frequency as the load increases. Thus, the switching frequency can only drop below the upper threshold of the audible range at high load conditions that are associated with when exceptional operating circumstances exist, such as a short circuit, initial power up, or sudden turn off situations. U.S. Pat. No. 7,202,609 is not directed to audible noise reduction in switch-mode power supplies under normal (non-exceptional) operating conditions where the switching frequency continuously decreases as the load decreases.
Therefore, there exists a need for a system and method for audible noise reduction in switch-mode power supplies with variable switching frequency where the switching frequency continuously decreases as the load decreases.